This invention relates to an automatic focusing apparatus for use in an optical system including a lens, more particularly, an optical system for observing an object, such as a microscope, or an industrial camera, such as a projection camera, e.g. a reduction camera, or the like.
When a substrate coated with sensitive material is positioned at the focus of a reduction camera with high resolution or the like, an apparatus for positioning the surface of the substrate at the desired focusing position, by moving it in response to a surface condition, e.g. concave and convex surface or undulated surface of the substrate, is required.
A prior construction of a focusing apparatus has been contemplated as illustrated in FIG. 1. The apparatus comprises detecting means, which utilizes an image field of a camera, for measuring the distance between the surface of the substrate and the detecting means by injecting a gas thereinto, a base for mounting the substrate (or sample), which is movable in a direction parallel to the optical axis of the camera, and a driving source and control system for moving the base in response to an output of the detecting means.
The principle of the above-mentioned detecting means for measuring the distance between the surface of the substrate and the detecting means is almost similar to that of an air micrometer. In FIG. 1, a lens housing 101 of the camera is divided by a lens 102 for reduction, and is provided with a detector 104 which surrounds a light guiding space of lens 102 at the side of the substrate 106. The detector 104 is provided with a blast hole 103, through which the gas, e.g. air having a constantly controlled pressure, is jetted, as shown with an arrow in FIG. 1, and an outlet 115 at the lower end of the detector 104. The outlet 115 has almost the same shape as that of the image field of the lens 102.
According to such a construction, the distance between the detector 104 and the substrate 106 is in a constant relation with the pressure within the detector 104 as shown in FIG. 2. Thus, the distance therebetween can be detected by measuring the pressure within the detector 104. In this case, the pressure within the detector 104 can be measured by a pressure gauge, e.g. a manometer 111, by passing a part of the gas out of the detecting means 104 through a measuring hole 105.
The focusing adjustment according to the above-mentioned focusing apparatus is hereinunder explained. First, a photoelectric cell 112 for detecting the position of a liquid surface of the manometer 111 is provided at the position of the liquid surface of the manometer which position represents the pressure value corresponding to the focal distance of the lens 102. In response to an output signal derived from the photoelectric cell 112, an electromagnetic valve 114 for supplying gas and an electromagnetic valve 113 for exhausting gas can be opened or shut, wherein the electromagnetic valve 114 for supplying the gas is connected to a suitable supply source (which is not shown in the drawing).
The substrate 106 is fixedly mounted on the base 107 which has been supported by a flexible parallel guide 108 such that it can be moved in the direction parallel to the optical axis of the lens 102. The base 107 is provided with a bellows 109 being a driving means, which is connected to the electromagnetic valves 113 and 114 through a supply and exhaust orifice 110.
The pressure within the detector 104 is lowered as the distance between the substrate 106 and the lens 102, or the detector 104, becomes greater than the predetermined distance value, so that the liquid surface of the manometer 111 is shifted. The shift of the liquid surface can be detected by the photoelectric cell 112, and then, the electromagnetic valve 114 can be opened responsive to the output of the photoelectric cell 112. Thus, the pressure within the bellows 109 is increased, so that the base 107 is moved nearer to the lens 102.
As the base 107 is moved, and the gap distance between the substrate 106 and the detector 104 reaches the predetermined distance value, then the pressure within the detector 104 indicates the predetermined pressure value and the electromagnetic valve 114 is shut. On the other hand, as the gap distance between the substrate 106 and the lens 102 is decreased, the pressure within the detector 104 is increased. Thus, the electromagnetic valve 113 for exhaust is opened and the base 107 recedes from the lens 102.
As stated above, by detecting the distance between the substrate and the lens in utilizing the image field of the substrate, by always maintaining the surface of the substrate at the predetermined position for focusing, the focusing adjustment can be done.
The gas, which is injected from the supply source into the detector 104 through the blast hole 103, has been considered to be air or the like, having a constant pressure obtained through a conventionally used pressure regulator. Instead of the above-mentioned manometer, the pressure measurement has also been considered to be accomplished by detecting the pressure difference between the pressure and the atmospheric pressure with a differential pressure transducer, which converts the predetermined focusing position into the electrical signal to open or shut the electromagnetic valves responsive to an amplified signal. The differential pressure transducer has been considered as a piezoelectric converter, which detects the pressure difference between two fluids as a displacement amount of the bellows, and which further converts its displacement into an electrical amount by means of a strain gauge or the like. Moreover, when it is unnecessary to consider the fluctuation of the atmospheric pressure of the circumference, it has been considered appropriate to use a piezoelectric converter capable of measuring the absolute pressure.
Further, the driving system of the above-mentioned focusing apparatus has been considered not only to move the base but also to move the lens housing 101. In this case, a servomotor or the like, instead of the bellows 109, has been used as the driving means, or it has been considered to regulate the distance between the lens and the substrate in a mechanism which has been supported by a piezoelectric cell.
In the focusing apparatus as detailed above, the gas is jetted into the optical path of the camera or the like, since the light guiding space of the lens 102 is utilized as a detector. In a method of supplying the gas into the detector, it has been considered to be effective to supply the gas in a tangential flow and to use it in a turning flow, so that the density distribution of the gas within the detector may be uniform. A supporting mechanism, such as a slide bearing, a roll bearing or the like, has been considered for the parallel movement mechanism in the direction of the optical axis, instead of the flexible parallel guide.
Furthermore, another focusing apparatus, which directly moves the driving system by utilizing the detected pressure, has been considered. Such an apparatus comprises a structure which leads the detected pressure from the measuring hole of the detector into the bellows, and thereby, moves the base mounting the substrate. In this case, when the gap distance between the lens and the substrate is narrowed, the detected pressure within the detector is increased. Thus, since the pressure within the bellows is increased thereby, the base is moved in a downward direction. On the other hand, as the gap distance is increased, the detected pressure is decreased. Thus, the base is moved in an upward direction since the bellows shrinks. As stated above, the focusing adjustment can be done since the surface of the substrate can be always supported at an identified position, and thus, the initially established position can be determined as the focusing position.
The driving mechanism of this apparatus is the same as that of the already-mentioned apparatus. Also, it has been considered not only to move the base but also to move the lens housing.
As detailed above, focusing apparatuses with different driving mechanisms, which utilize detectors such as air micrometers, have been considered. According to such apparatus, it is possible to precisely support the focusing position after the focusing position has been established.
However, in the case of the former focusing apparatus already mentioned, its focusing adjustment must be done by always watching the pressure of the gas supply source so that the balanced position of the base may not be influenced by the pressure fluctuation of the gas supply source. Further, in the case of the latter mentioned focusing apparatus, which directly moves the driving mechanism by utilizing the detected pressure, it is necessary to carefully increase or decrease the load of the base, and to carefully move the base up and down, in order to correctly establish the balanced position at the focusing position.
Accordingly, these prior focusing apparatuses have a common disadvantage in that the adjustment for initially determining the focusing position before operation requires a long time and a lot of effort to position the substrate surface at the focusing position.